1. Field of the Invention
The present invention relates to an element analyzing method using fluorescent X-rays, and more particularly to a contaminating-element analyzing method which make possible precise identification of contaminating elements and precise calculation of concentrations thereof.
2. Related Background Art
The X-ray fluorescence analysis has been used heretofore as a nondestructive element analysis for an object to be measured (sample). The total reflection X-ray fluorescence analysis was also developed to increase the sensitivity and is under study for applications to metal contamination control in semiconductor process (Ayako, SHIMAZAKI and Kuniharu, MIYAZAKI: NIKKEI MICRO DEVICE, p148 No. 86, August 1992). Among the total reflection X-ray fluorescence analyses, the energy dispersive X-ray fluorescence analysis is effective for measuring a spectrum in a wide energy range and therefore enables simultaneous analysis of multiple elements through a single solid-state detector (SSD) disposed immediately above the sample. Also, since the energy dispersive X-ray fluorescence analysis needs no analyzing crystal, the SSD can be set closer to the sample. For that reason, the energy dispersive method has a feature of higher sensitivity than that in the wave dispersive method in X-ray fluorescence analysis.
Further, the energy-dispersive total reflection X-ray fluorescence analysis is inferior in spectral resolution to the wavelength-dispersive analysis, providing a smaller total count number. Therefore, the total reflection method is likely to be affected by statistical error of counting.
In the total reflection X-ray fluorescence analysis there are various impeding peaks in addition to peaks from characteristic X-rays of elements in an initially measured waveform. That is, observed in the measured waveform are peaks specific to the SSD, for example sum peaks and escape peaks, or diffraction peaks if the Bragg condition is satisfied by angles among the primary X-ray, the sample and the SSD.
In addition to the peaks as described above, there appears a peak due to Rayleigh scattering, in which a characteristic X-ray of a material of rotating target as X-ray source is Rayleigh-scattered by the sample. For example, if the anticathode is made of a target material of W which has a high excitation efficiency for transition metals, a peak is detected at 9.671 keV of W-L.sub..beta.1. Additionally, when the characteristic X-ray of the anticathode material impinges on the sample, a part of the X-ray is subjected to Compton scattering, thereby to lose a part of energy when detected. As a result, the two types of scattering peaks (Rayleigh scattering peak and Compton scattering peak) are detected in the form of peak trailing on the low energy side (see FIGS. 2A and 2B).
Since the scattering peaks each have a broad spread on the low energy side, they are impeding peaks to the K.sub..alpha. peak (8.63 keV) of Zn, as shown in (a) in FIG. 3. The influence of such impeding peaks becomes outstanding in case of contamination in low concentration, which could be an error factor in identification of each analyzing element peak and in calculation of intensity thereof. In the case where the intensity of each contaminating element is calculated by the ROI (Region of Interest) method for example, it could become difficult to set a region for calculation. Also, if waveform separation is carried out based on the nonlinear optimization method, treatment of background could be difficult, which in turn results in making identification of peaks and calculation of intensities thereof difficult. This problem is caused by the waveform separation in which data of asymmetric waveform trailing on the low energy side due to the Compton scattering is replaced by a symmetric Gaussian function.